A photovoltaic solar cell represents a planar semiconductor component, in which by absorption of incident electromagnetic radiation pairs of charge carriers are generated and subsequently separated at a pn-junction such that a potential develops between at least two electric contacting points of the solar cell and electric power can be tapped.
The pn-junction can be realized such that in a semiconductor substrate with a base doping, by way of diffusion of a doping substance of an emitter doping opposite the base doping, a respective emitter area is formed such that a pn-junction develops between the emitter area and the base-doped area of the semiconductor substrate.
It is also known to embody the emitter by applying one or more layers on a base substrate, particularly by applying an emitter layer made from amorphous silicon onto a base substrate comprising monocrystalline silicon. Here too, the emitter layer comprises a doping type opposite the one of the base so that a pn-junction develops between the emitter and the base. Due to the fact that the amorphous silicon layer of the emitter exhibits a band gap different from the crystalline silicon of the base, here a so-called hetero-pn junction forms so that a so-called hetero-emitter is given.
If the base substrate and the amorphous silicon layer have the same doping type, however different doping concentrations, a hetero-junction develops as well, in this case a so-called “high-low junction”. Such a hetero-junction is used to form hetero-contacts:
Different physical types of contacting are also known for the contacting of semiconductor areas: typically a metallic contacting structure is applied directly or indirectly on the semiconductor area to be contacted. Here, particularly the formation of an ohmic contact and a Schottky-contact is known. MOS/MIS-contacts are also known as a particular embodiment of a hetero-contact. A particular embodiment of MOS-MIS contacts exhibits the following design: substrate/tunnel oxide doped poly-Si layer. Such types of contacts are known for semiconductors and described for example in Peter Warfel, Physics of Solar Cells: From Principles to New Concepts, 2005, Weinheim: Wiley-VCH. (hetero-contact: chapter 6.6, p. 127ff; Schottky-contact: chapter 6.7.1, p. 131f; MIS-contact: chapter 6.7.2, p. 132) and Sze, S. M., Semiconductor devices: Physics and Technology, 1985, New York: John Wiley & sons, (MOS-contact: chapter 5.4, p. 186; metal-semiconductor contact: chapter 5.1, p. 160ff.)
Hetero-junctions are classically called junctions of materials having different band gaps. However, hetero-contacts may also be formed with an arrangement of a tunnel layer between the semiconductor substrate and the hetero-junction layer, for example as a substrate/tunnel-oxide/silicon-containing layer and/or MIS-contacts as described above. The name “hetero-junction” is used in this application in a comprehensive sense. The “hetero”-features of the hetero-junction can therefore be based on different band gaps between the semiconductor substrate and the hetero-junction layer and/or between the tunnel layer and the hetero-junction layer.
The term “hetero-junction” comprises in this application, as mentioned at the outset, both junctions with layers of different doping types, particularly to form hetero-emitters, as well as junctions with layers of the same doping type, particularly for the formation of hetero-contacts.
Analogous to the definition introduced with regards to the emitters, in the present case any contacts not representing hetero-contacts are called homo-contacts.
Photovoltaic solar cells having a hetero-emitter and/or a hetero-contact, i.e. at least one hetero-junction, are called hetero-solar cells. Such hetero-solar cells are known from DE 10 2008 030 693 A1 and US 2012/0055547 A1, for example.